In the initial year of work on this project an important aspect of ADH-action has been resolved using differential interference-contrast and fluorescence microscopy of living amphibian urinary bladders. The hormone appears to induce a relaxation of terminal web microfilaments underlying the mucosal membrane of granular cells. This action increases the compliance of the mucosal membrane so that the ridge-like structure of membrane microvilli is converted to finger-like projections. As a consequence, during water flow from mucosa to serosa, the swelling of cells is preferentially located in the apical aspect of the granular cell cytoplasm. This has been documented by optical section analysis of cells during ADH-induced flow. Restriction of expansion to this end of the cell effectively protects mitochondria from swelling. Establishing that this is the case has been done with the use of the fluorescent dye, dimethylaminostyrylmethylpyridiniumiodine (DASPMI), which specifically stains mitochondria and reveals their existence as long threads in a network within the cell. In the forthcoming year, major emphasis will be on clarification of these findings with respect to how much dilution of cytoplasm actually occurs and ADH-induced flow and how the process is involved with cytoskeletal elements.